A canister is probably going to be the simplest out of the box solution.
You could build a filter box inside the tank. I would use glass because it will adhere better with silicone than plexiglass. If you prefer to work with plexiglass, I would build a whole box inside the tank and then hang it or use magnets to keep it at the desired level.
The HMF could be a good option. You can add strips of glass to install them in the back corners and then use water pumps to provide flow.
If looks don't matter a ton, then a "dump filter" - basically a wet dry on top of the tank - is an option.
You could build a filter box inside the tank. I would use glass because it will adhere better with silicone than plexiglass. If you prefer to work with plexiglass, I would build a whole box inside the tank and then hang it or use magnets to keep it at the desired level.
The HMF could be a good option. You can add strips of glass to install them in the back corners and then use water pumps to provide flow.
If looks don't matter a ton, then a "dump filter" - basically a wet dry on top of the tank - is an option.